Power machineries, such as hydraulic excavators, cranes, wheel loaders, and bulldozers, (in the present specification and claims, these power machineries (heavy machineries) are collectively called “operating machines”) have been used for civil engineering works, construction works, and the like. Taking, the hydraulic excavator as an example, the hydraulic excavator is configured such that: a revolving super structure (structure) is provided on the upper portion of a base carrier; and the revolving super structure includes an engine, a driver's seat, an arm having a tip end on which a bucket is provided, a boom coupled to the arm, and the like. Therefore, the revolving super structure is a large heavy structure. By manipulating a remote control valve at the driver's seat during operations, the revolving super structure is caused to swing on the upper portion of the base carrier. In addition, by manipulating the boom and the like, various operations are performed by the bucket provided at the tip end of the arm.
The revolving super structure is caused to swing by a driving device configured to cause the revolving super structure to swing. In recent years, a device including a hydraulic motor and an electric motor has been proposed as the driving device.
One example of an operating machine including this type of driving device is an operating machine including a driving device which includes an hydraulic unit having a hydraulic motor as a driving source and an electric unit having an electric motor as a driving source, controls the electric motor by a controller and an inverter at the time of swing, and assists the hydraulic twit by the torque of the electric motor (see PTL 1, for example). According to this operating machine, when the driving device performs steady swing or decelerates, the electric motor is caused to perform a regenerative action, and regenerative electric power is stored in a capacitor. A control unit of the driving device calculates required torque at the time of the swing. When the required torque exceeds a set value, the electric motor outputs necessary torque. To be specific, maximum torque necessary as a whole is secured by causing the electric unit to assist the hydraulic unit. In this case, necessary torque is generated by adjusting the assist amount of the electric unit. The control unit is configured to control the output torque of the electric motor so as to shorten a relief time of a relief valve provided at a hydraulic motor circuit.
Further, another prior art is that: a construction machinery including a hybrid driving device having a driving force synthesis mechanism configured to synthesize the driving force of a hydraulic actuator and the driving force of a motor generator includes a communication valve (bypass valve) for effectively utilizing energy generated at the time of braking; and inertial energy of the revolving super structure is efficiently regenerated as electric energy by the motor generator (see PTL 2, for example). In this prior art, the relief valve incorporated in the hydraulic motor is used as a setting unit configured to set a ratio between the driving force of the hydraulic actuator and the driving force of the motor generator configured to cooperate with the hydraulic actuator.
Still another prior art is that: a pressure difference between both ports of the hydraulic actuator is detected; and a torque command is output to the motor generator, provided close to the hydraulic actuator, in accordance with the pressure difference (see PTL 3, for example). In this prior art, the revolving super structure is configured to be driven by the sum of driving and braking torques of the hydraulic motor and the motor generator. The relief valve configured to control highest driving pressure of the hydraulic motor at the time of driving and stopping is provided as an adjusting unit configured to perform adjustments such that the ratio of the output torque of the hydraulic motor at the time of the driving becomes larger than that at the time of the braking. The working pressure of the relief valve at the time of the start-up and acceleration is set to be higher than that at the time of the deceleration and stopping.